I use gameshark codes to steal rental Pokemon from Pokemon emerald's battle factory. Basically what I've done is go into the battle factory and rent 3 Pokemon. I then use the gameshark code to teleport out, keeping the rental Pokemon and giving them the junk Pokemon that was on me. Here are the codes which must be inputted:
Master Code:
9266FA6C97BD
905B5ED35F81
B76A68E5FAB1
Warp Code:
19C67C656689
94318B106757